Conventionally, providers of consumer services, such as, for example, utilities (i.e., gas and electricity), and the like, have been required to send a service representative to a consumer location in order to monitor service usage and to update the services required by the consumer when requested. This task is extremely labor intensive and costly. A possible alternative to conventional command, control and communication methods used by service providers could be a system that uses a modem in conjunction with existing telephone or cable infrastructure to accomplish certain functions, such as service selection and monitoring. However, there are many disadvantages attendant with such systems. For example, in many international locations, a telephone network is not a practical solution due to the lack of availability or access, or consumer concerns related to the cost associated with such a system. Another disadvantage associated with using telephone or cable infrastructure is that in most areas, cable or telephone infrastructures do not exactly overlay the same region as a given service provider. This is especially true in cases where the service provider, is covering a large national or international region. Moreover, in remote or difficult-to-access areas it may be impractical or even impossible to provide a land-based infrastructure, such as telephone lines or cable. Other alternatives have also been considered, however, these systems also suffer serious drawbacks due to their dependence on terrestrial infrastructure. For example, cellular networks and dedicated RF systems have been contemplated. However, these systems may require the incorporation of additional terrestrial infrastructure and do not always provide complete coverage, especially for remote and difficult-to-access areas. Thus, these systems suffer many of the same shortcomings as conventional telephone and cable systems.
In 1990, the United States Federal Communications Commission allocated spectrum in the VHF and UHF bands for Non-Voice, Non-Geosynchronous Mobile Satellite Services (NVNG-MSS). At the World Administrative Radio Conference in 1992, certain frequencies were allocated on a world-wide primary shared basis for the new NVNG-MSS. In particular, the 137-138 MHz, 148-150.05 MHz and 400.15-401 MHz frequency bands, all of which fall below 1 GHz, were allocated. The United States Federal Communications Commission, in its Report and Order, released in November, 1993, modified these frequency allocations slightly to comprise the bands 137-138 MHz and 400.15-401 MHz for space-to-Earth communications and 148-150.05 MHz and 399.9-400.05 MHz for Earth-to-space communications.
In connection with the allocation of transmission frequencies, the term "little" low earth orbit satellite (LLEO) system was originated. Low earth orbit (LEO) satellites are also contemplated for application at frequencies above 1 GHz, such as the so-called Iridium system, and these satellites have been named "big" low earth orbit satellites. Hence, the term "little" LEO satellite systems generally refers to systems in which radio transmission frequencies below 1 GHz have been allocated.
Geostationary satellites have been utilized for a number of years to provide intercontinental telecommunications services. A geostationary satellite is positioned so as to revolve around the earth at a speed equal to that of the rotation of the earth and at such a distance as to not: escape the earth's gravitational field; eventually drift toward the earth; or substantially move relative to its position with respect to the earth's surface. Thus, a geostationary satellite does not move relative to any position within its reach such that a receive/transmit antenna would need to "track" or move to follow the satellite's movement in order to achieve acceptable signal transmission or reception. The geostationary, or "Clarke," orbit is at 35,786 kilometers, or 22,247 miles above the surface of the earth. This distance requires a two-way time delay, at the speed of light, in the range of tenths of seconds to traverse. Geostationary satellites have proven extremely useful for wideband one-way communication signals, such as, for example, television signals, where absolute delay is not a problem.
The newly proposed LEO satellites for NVNG-MSS service will not suffer from the difficulty of real-time delay because LEOs are positioned much closer to the earth than geostationary satellites. For example, LEO satellites may be positioned in polar, north-south orbits at distances from 250 km to 1500 km from the earth's surface. These orbits may be inclined or not. An area of the surface of the earth covered by a particular satellite is generally referred to as the satellite's "footprint." Due to their proximity to the earth, LEO satellite footprints are generally smaller than those of geostationary satellites at the same power level. Referring to FIG. 1, there are shown sample "visibility" footprints for LEO satellites at 250 km and 750 km, assuming the satellite is standing still relative to the earth's surface. FIG. 1 is taken from the paper "Introduction of LEO Satellites Below 1 GHz Sharing in the Uplink Band," Technical Report RP 328 (Issue 2), by K. Brown, September, 1992, the disclosure of which is herein incorporated by reference in its entirety. In actuality, the LEO satellites will be moving north-to-south in one pass of the North American continent and south-to-north in a second pass. Their passes will cover different footprints with each pass as the earth rotates beneath them. As another consequence of their proximity to the earth, LEO satellites travel at such speeds, relative to that of the earth, that their footprints may be traversed within periods as short as, or shorter than, thirty minutes, depending upon their altitudes.
The first LEO communications satellite, known as Telstar, was only capable of providing communication between Europe and the United States during a thirty-six minute window during which its footprint was visible from both sides of the Atlantic Ocean. Short footprint durations have led to a move away from what are seen as the disadvantageous LEOs and toward geostationary orbit satellites which provide twenty-four hour service in its footprint. However, LEOs remain of interest due to the increasing need for relatively narrow-band, i.e., slow speed, non-voice data communications.
Little LEO satellite systems are optimally suited for handling bursty low data rate messages, typically in the range of 6-256 bytes, with the system being optimized for small amounts of data, i.e., not voice or video. LLEO systems generally employ a packet switched system that is inherently more efficient than a switched circuit network. Big LEOs are examples of systems that use switched circuit networks.
LLEO satellites are constantly circling the earth. Orbcomm and Starsys are two companies that are currently in the process of obtaining U.S. Federal Communications Commission licenses for operating LLEO satellites. Each of these companies intends to put up a constellation of LLEO satellites (24 for Starsys and 36 for Orbcomm) which will be placed in inclined polar orbits. Groups of satellites will be separated into different planes and equally spaced to provide constant coverage of the earth. For example, once the Orbcomm system is fully deployed, there will be constant real-time coverage of the entire globe. In fact, three satellites will always be in view at any given time at any given ground location. Referring briefly to FIG. 2, a representative coverage of twenty-four LEO satellites at a given time is shown. This figure is taken from Starsys Global Positioning, Inc.'s amended application to the U.S. Federal Communications Commission. From FIG. 2 it can be seen that virtually the entire earth's surface may be covered by twenty-four satellites. Moreover, according to one of Starsys' own charts, shown herein at FIG. 3, depending on the latitude, twenty-four LEO satellites will, in fact, predictably cover 100% of the earth's surface. In other words, one satellite or another will be visible from forty-eight degrees latitude 100% of the time. Similarly, Orbcomm's system using thirty-six LEO satellites will provide coverage wherein at least three satellites are visible to any given area 100% of the time. The ability to realize real-time coverage facilitates a wide variety of functions for systems that use LLEOs.
Each LLEO system currently contemplated can operate in two primary modes: real-time, so-called "bent pipe"; or store and forward. Bent pipe mode means that for mobile transceivers and Earth stations concurrently within the satellite's footprint, data communication can occur directly from the mobile transceiver to the Earth station gateway. In other words, operation in bent pipe mode requires that the satellite antenna footprint be capable of "seeing" both the transceiver and the Earth station at the same time. Because the antenna footprint of LLEO satellites are several thousand miles in diameter, most regions of the United States will be able to operate in bent pipe mode, thereby operating in virtual real-time. Store and forward services means providing the satellite with data storage memories for storing data received from mobile transceivers for later transmission to an Earth station as the satellite moves from the footprint in which the mobile transceiver is located to one in which the desired Earth station is located.
While LEO satellite systems are contemplated, their utilization for various applications is still in the exploratory stages of development. There remains a need, notwithstanding prior ground-based systems, such as, for example, telephone and cable, for an efficient, reliable path for exchanging data, command and control signals between a consumer and a supplier of services, especially in remote and difficult-to-access areas.